DNA Binds to Cell Membranes and is Internalized
In the 1970's, several investigators reported the binding of high molecular weight DNA to cell membranes (Lerner, R. A., W. Meinke, and D. A. Goldstein. 1971. “Membrane-associated DNA in the cytoplasm of diploid human lymphocytes”. Proc. Natl. Acad. Sci. USA 68:1212; Agrawal, S. K., R. W. Wagner, P. K. McAllister, and B. Rosenberg. 1975. “Cell-surface-associated nucleic acid in tumorigenic cells made visible with platinum-pyrimidine complexes by electron microscopy”. Proc. Natl. Acad. Sci. USA 72:928). In 1985, Bennett et al. presented the first evidence that DNA binding to lymphocytes is similar to a ligand receptor interaction: binding is saturable, competitive, and leads to DNA endocytosis and degradation into oligonucleotides (Bennett, R. M., G. T. Gabor, and M. M. Merritt. 1985. “DNA binding to human leukocytes. Evidence for a receptor-mediated association, internalization, and degradation of DNA”. J. Clin. Invest. 76:2182). Like DNA, oligodeoxyribonucleotides (ODNs) are able to enter cells in a saturable, sequence independent, and temperature and energy dependent fashion (reviewed in Jaroszewski, J. W., and J. S. Cohen. 1991. “Cellular uptake of antisense oligodeoxynucleotides”. Advanced Drug Delivery Reviews 6:235; Akhtar, S., Y. Shoji, and R. L. Juliano. 1992. “Pharmaceutical aspects of the biological stability and membrane transport characteristics of antisense oligonucleotides”. In: Gene Regulation: Biology of Antisense RNA and DNA. R. P. Erickson, and J. G. Izant, eds. Raven Press, Ltd. New York, pp. 133; and Zhao, Q., T. Waldschmidt, E. Fisher, C. J. Herrera, and A. M. Krieg., 1994. “Stage specific oligonucleotide uptake in murine bone marrow B cell precursors”. Blood, 84:3660). No receptor for DNA or ODN uptake has yet been cloned, and it is not yet clear whether ODN binding and cell uptake occurs through the same or a different mechanism from that of high molecular weight DNA.
Lymphocyte ODN uptake has been shown to be regulated by cell activation. Spleen cells stimulated with the B cell mitogen LPS had dramatically enhanced ODN uptake in the B cell population, while spleen cells treated with the T cell mitogen Con A showed enhanced ODN uptake by T but not B cells (Krieg, A. M., F. Gmelig-Meyling, M. F. Gourley, W. J. Kisch, L. A. Chrisey, and A. D. Steinberg. 1991. “Uptake of oligodeoxyribonucleotides by lymphoid cells is heterogeneous and inducible”. Antisense Research and Development 1:161).
Immune Effects of Nucleic Acids
Several polynucleotides have been extensively evaluated as biological response modifiers. Perhaps the best example is poly (I,C) which is a potent inducer of IFN production as well as a macrophage activator and inducer of NK activity (Talmadge, J. E., J. Adams, H. Phillips, M. Collins, B. Lenz, M. Schneider, E. Schlick, R. Ruffmann, R. H. Wiltrout, and M. A. Chirigos. 1985. “Immunomodulatory effects in mice of polyinosinic-polycytidylic acid complexed with poly-L-lysine and carboxymethylcellulose”. Cancer Res. 45:1058; Wiltrout, R. H., R. R. Salup, T. A. Twilley, and J. E. Talmadge. 1985. “Immunomodulation of natural killer activity by polyribonucleotides”. J. Biol. Resp. Mod. 4:512; Krown, S. E. 1986. “Interferons and interferon inducers in cancer treatment”. Sem. Oncol. 13:207; and Ewel, C. H., S. J. Urba, W. C. Kopp, J. W. Smith II, R. G. Steis, J. L. Rossio, D. L. Longo, M. J. Jones, W. G. Alvord, C. M. Pinsky, J. M. Beveridge, K. L. McNitt, and S. P. Creekmore. 1992. “Polyinosinic-polycytidylic acid complexed with poly-L-lysine and carboxymethylcellulose in combination with interleukin-2 in patients with cancer: clinical and immunological effects”. Canc. Res. 52:3005). It appears that this murine NK activation may be due solely to induction of IFN-β secretion (Ishikawa, R., and C. A. Biron. 1993. “IFN induction and associated changes in splenic leukocyte distribution”. J. Immunol. 150:3713). This activation was specific for the ribose sugar since deoxyribose was ineffective. Its potent in vitro antitumor activity led to several clinical trials using poly (I,C) complexed with poly-L-lysine and carboxymethylcellulose (to reduce degradation by RNAse) (Talmadge, J. E., et al., 1985. cited supra; Wiltrout, R. H., et al., 1985. cited supra); Krown, S. E., 1986. cited supra); and Ewel, C. H., et al., 1992. cited supra). Unfortunately, toxic side effects have thus far prevented poly (I,C) from becoming a useful therapeutic agent.
Guanine ribonucleotides substituted at the C8 position with either a bromine or a thiol group are B cell mitogens and may replace “B cell differentiation factors” (Feldbush, T. L., and Z. K. Ballas. 1985. “Lymphokine-like activity of 8-mercaptoguanosine: induction of T and B cell differentiation”. J. Immunol. 134:3204; and Goodman, M. G. 1986. “Mechanism of synergy between T cell signals and C8-substituted guanine nucleosides in humoral immunity: B lymphotropic cytokines induce responsiveness to 8-mercaptoguanosine”. J. Immunol. 136:3335). 8-mercaptoguanosine and 8-bromoguanosine also can substitute for the cytokine requirement for the generation of MHC restricted CTL (Feldbush, T. L., 1985. cited supra), augment murine NK activity (Koo, G. C., M. E. Jewell, C. L. Manyak, N. H. Sigal, and L. S. Wicker. 1988. “Activation of murine natural killer cells and macrophages by 8-bromoguanosine”. J. Immunol. 140:3249), and synergize with IL-2 in inducing murine LAK generation (Thompson, R. A., and Z. K. Ballas. 1990. “Lymphokine-activated killer (LAK) cells. V. 8-Mercaptoguanosine as an IL-2-sparing agent in LAK generation”. J. Immunol. 145:3524). The NK and LAK augmenting activities of these C8-substituted guanosines appear to be due to their induction of IFN (Thompson, R. A., et al. 1990. cited supra). Recently, a 5′ triphosphorylated thymidine produced by a mycobacterium was found to be mitogenic for a subset of human γδ T cells (Constant, P., F. Davodeau, M.-A. Peyrat, Y. Poquet, G. Puzo, M. Bonneville, and J.-J. Foumie. 1994. “Stimulation of human γδ T cells by nonpeptidic mycobacterial ligands” Science 264:267). This report indicated the possibility that the immune system may have evolved ways to preferentially respond to microbial nucleic acids.
Several observations suggest that certain DNA structures may also have the potential to activate lymphocytes. For example, Bell et al. reported that nucleosomal protein-DNA complexes (but not naked DNA) in spleen cell supernatants caused B cell proliferation and immunoglobulin secretion (Bell, D. A., B. Morrison, and P. VandenBygaart. 1990. “Immunogenic DNA-related factors”. J. Clin. Invest. 85:1487). In other cases, naked DNA has been reported to have immune effects. For example, Messina et al. have recently reported that 260 to 800 bp fragments of poly (dG)•(dC) and poly (dG•dC) were mitogenic for B cells (Messina, J. P., G. S. Gilkeson, and D. S. Pisetsky. 1993. “The influence of DNA structure on the in vitro stimulation of murine lymphocytes by natural and synthetic polynucleotide antigens”. Cell. Immunol. 147:148). Tokunaga, et al. have reported that dG•dC induces IFN-γ and NK activity (Tokunaga, S. Yamamoto, and K. Namba. 1988. “A synthetic single-stranded DNA, poly(dG,dC), induces interferon-α/β and -γ, augments natural killer activity, and suppresses tumor growth” Jpn. J. Cancer Res. 79:682). Aside from such artificial homopolymer sequences, Pisetsky et al. reported that pure mammalian DNA has no detectable immune effects, but that DNA from certain bacteria induces B cell activation and immunoglobulin secretion (Messina, J. P., G. S. Gilkeson, and D. S. Pisetsky. 1991. “Stimulation of in vitro murine lymphocyte proliferation by bacterial DNA”. J. Immunol. 147:1759). Assuming that these data did not result from some unusual contaminant, these studies suggested that a particular structure or other characteristic of bacterial DNA renders it capable of triggering B cell activation. Investigations of mycobacterial DNA sequences have demonstrated that ODN which contain certain palindrome sequences can activate NK cells (Yamamoto, S., T. Yamamoto, T. Kataoka, E. Kuramoto, O. Yano, and T. Tokunaga. 1992. “Unique palindromic sequences in synthetic oligonucleotides are required to induce INF and augment INF-mediated natural killer activity”. J. Immunol. 148:4072; Kuramoto, E., O. Yano, Y. Kimura, M. Baba, T. Makino, S. Yamamoto, T. Yamamoto, T. Kataoka, and T. Tokunaga. 1992. “Oligonucleotide sequences required for natural killer cell activation”. Jpn. J. Cancer Res. 83:1128).
Several phosphorothioate modified ODN have been reported to induce in vitro or in vivo B cell stimulation (Tanaka, T., C. C. Chu, and W. E. Paul. 1992. “An antisense oligonucleotide complementary to a sequence in Iγ2b increases γ2b germline transcripts, stimulates B cell DNA synthesis, and inhibits immunoglobulin secretion”. J. Exp. Med. 175:597; Branda, R. F., A. L. Moore, L. Mathews, J. J. McCormack, and G. Zon. 1993. “Immune stimulation by an antisense oligomer complementary to the rev gene of HIV-1”. Biochem. Pharmacol. 45:2037; McIntyre, K. W., K. Lombard-Gillooly, J. R. Perez, C. Kunsch, U. M. Sarmiento, J. D. Larigan, K. T. Landreth, and R. Narayanan. 1993. “A sense phosphorothioate oligonucleotide directed to the initiation codon of transcription factor NFκB T65 causes sequence-specific immune stimulation”. Antisense Res. Develop. 3:309; and Pisetsky, D. S., and C. F. Reich. 1993. “Stimulation of murine lymphocyte proliferation by a phosphorothioate oligonucleotide with antisense activity for herpes simplex virus”. Life Sciences 54:101). These reports do not suggest a common structural motif or sequence element in these ODN that might explain their effects.
The CREB/ATF Family of Transcription Factors and their Role in Replication
The cAMP response element binding protein (CREB) and activating transcription factor (ATF) or CREB/ATF family of transcription factors is a ubiquitously expressed class of transcription factors of which 11 members have so far been cloned (reviewed in de Groot, R. P., and P. Sassone-Corsi: “Hormonal control of gene expression: Multiplicity and versatility of cyclic adenosine 3′,5′-monophosphate-responsive nuclear regulators”. Mol. Endocrin. 7:145, 1993; Lee, K. A. W., and N. Masson: “Transcriptional regulation by CREB and its relatives”. Biochim. Biophys. Acta 1174:221, 1993.). They all belong to the basic region/leucine zipper (bZip) class of proteins. All cells appear to express one or more CREB/ATF proteins, but the members expressed and the regulation of mRNA splicing appear to be tissue-specific. Differential splicing of activation domains can determine whether a particular CREB/ATF protein will be a transcriptional inhibitor or activator. Many CREB/ATF proteins activate viral transcription, but some splicing variants which lack the activation domain are inhibitory. CREB/ATF proteins can bind DNA as homo- or hetero-dimers through the cAMP response element, the CRE, the consensus form of which is the unmethylated sequence TGACGTC (binding is abolished if the CpG is methylated) (Iguchi-Ariga, S. M. M., and W. Schaffner: “CpG methylation of the cAMP-responsive enhancer/promoter sequence TGACGTCA abolishes specific factor binding as well as transcriptional activation”. Genes & Develop. 3:612, 1989.).
The transcriptional activity of the CRE is increased during B cell activation (Xie, H. T. C. Chiles, and T. L. Rothstein: “Induction of CREB activity via the surface Ig receptor of B cells”. J. Immunol. 151:880, 1993.). CREB/ATF proteins appear to regulate the expression of multiple genes through the CRE including immunologically important genes such as fos, jun B, Rb-1, IL-6, IL-1 (Tsukada, J., K. Saito, W. R. Waterman, A. C. Webb, and P. E. Auron: “Transcription factors NF-IL6 and CREB recognize a common essential site in the human prointerleukin 1β gene”. Mol. Cell. Biol. 14:7285, 1994; Gray, G. D., O. M. Hernandez, D. Hebel, M. Root, J. M. Pow-Sang, and E. Wickstrom: “Antisense DNA inhibition of tumor growth induced by c-Ha-ras oncogene in nude mice”. Cancer Res. 53:577, 1993), IFN-β (Du, W., and T. Maniatis: “An ATF/CREB binding site protein is required for virus induction of the human interferon B gene”. Proc. Natl. Acad. Sci. USA 89:2150, 1992), TGF-β1 (Asiedu, C. K., L. Scott, R. K. Assoian, M. Ehrlich: “Binding of AP-1/CREB proteins and of MDBP to contiguous sites downstream of the human TGF-B1 gene”. Biochim. Biophys. Acta 1219:55, 1994.), TGF-β2, class II MHC (Cox, P. M., and C. R. Goding: “An ATF/CREB binding motif is required for aberrant constitutive expression of the MHC class II DRa promoter and activation by SV40 T-antigen”. Nucl. Acids Res. 20:4881, 1992.), E-selectin, GM-CSF, CD-8α, the germline Igα constant region gene, the TCR Vβ gene, and the proliferating cell nuclear antigen (Huang, D., P. M. Shipman-Appasamy, D. J. Orten, S. H. Hinrichs, and M. B. Prystowsky: “Promoter activity of the proliferating-cell nuclear antigen gene is associated with inducible CRE-binding proteins in interleukin 2-stimulated T lymphocytes”. Mol. Cell. Biol. 14:4233, 1994.). In addition to activation through the cAMP pathway, CREB can also mediate transcriptional responses to changes in intracellular Ca++ concentration (Sheng, M., G. McFadden, and M. E. Greenberg: “Membrane depolarization and calcium induce c-fos transcription via phosphorylation of transcription factor CREB”. Neuron 4:571, 1990).
The role of protein-protein interactions in transcriptional activation by CREB/ATF proteins appears to be extremely important. There are several published studies reporting direct or indirect interactions between NFKB proteins and CREB/ATF proteins (Whitley, et. al., (1994) Mol. & Cell. Biol. 14:6464; Cogswell, et al., (1994) J. Immun. 153:712; Hines, et al., (1993) Oncogene 8:3189; and Du, et al., (1993) Cell 74:887. Activation of CREB through the cyclic AMP pathway requires protein kinase A (PKA), which phosphorylates CREB341 on ser133 and allows it to bind to a recently cloned protein, CBP (Kwok, R. P. S., J. R. Lundblad, J. C. Chrivia, J. P. Richards, H. P. Bachinger, R. G. Brennan, S. G. E. Roberts, M. R. Green, and R. H. Goodman: “Nuclear protein CBP is a coactivator for the transcription factor CREB”. Nature 370:223, 1994; Arias, J., A. S. Alberts, P. Brindle, F. X. Claret, T. Smea, M. Karin, J. Feramisco, and M. Montminy: “Activation of cAMP and mitogen responsive genes relies on a common nuclear factor”. Nature 370:226, 1994.). CBP in turn interacts with the basal transcription factor TFIIB causing increased transcription. CREB also has been reported to interact with dTAFII 110, a TATA binding protein-associated factor whose binding may regulate transcription (Ferreri, K., G. Gill, and M. Montminy: “The cAMP-regulated transcription factor CREB interacts with a component of the TFIID complex”. Proc. Natl. Acad. Sci. USA 91:1210, 1994.). In addition to these interactions, CREB/ATF proteins can specifically bind multiple other nuclear factors (Hoeffler, J. P., J. W. Lustbader, and C.-Y. Chen: “Identification of multiple nuclear factors that interact with cyclic adenosine 3′,5′-monophosphate response element-binding protein and activating transcription factor-2 by protein-protein interactions”. Mol. Endocrinol. 5:256, 1991) but the biologic significance of most of these interactions is unknown. CREB is normally thought to bind DNA either as a homodimer or as a heterodimer with several other proteins. Surprisingly, CREB monomers constitutively activate transcription (Krajewski, W., and K. A. W. Lee: “A monomeric derivative of the cellular transcription factor CREB functions as a constitutive activator”. Mol. Cell. Biol. 14:7204, 1994.).
Aside from their critical role in regulating cellular transcription, it has recently been shown that CREB/ATF proteins are subverted by some infectious viruses and retroviruses, which require them for viral replication. For example, the cytomegalovirus immediate early promoter, one of the strongest known mammalian promoters, contains eleven copies of the CRE which are essential for promoter function (Chang, Y.-N., S. Crawford, J. Stall, D. R. Rawlins, K.-T. Jeang, and G. S. Hayward: “The palindromic series I repeats in the simian cytomegalovirus major immediate-early promoter behave as both strong basal enhancers and cyclic AMP response elements”. J. Virol. 64:264, 1990). At least some of the transcriptional activating effects of the adenovirus E1A protein, which induces many promoters, are due to its binding to the DNA binding domain of the CREB/ATF protein, ATF-2, which mediates E1A inducible transcription activation (Liu, F., and M. R. Green: “Promoter targeting by adenovirus E1a through interaction with different cellular DNA-binding domains”. Nature 368:520, 1994). It has also been suggested that E1A binds to the CREB-binding protein, CBP (Arany, Z., W. R. Sellers, D. M. Livingston, and R. Eckner: “E1A-associated p300 and CREB-associated CBP belong to a conserved family of coactivators”. Cell 77:799, 1994). Human T lymphotropic virus-I (HTLV-1), the retrovirus which causes human T cell leukemia and tropical spastic paresis, also requires CREB/ATF proteins for replication. In this case, the retrovirus produces a protein, Tax, which binds to CREB/ATF proteins and redirects them from their normal cellular binding sites to different DNA sequences (flanked by G- and C-rich sequences) present within the HTLV transcriptional enhancer (Paca-Uccaralertkun, S., L.-J. Zhao, N. Adya, J. V. Cross, B. R. Cullen, I. M. Boros, and C.-Z. Giam: “In vitro selection of DNA elements highly responsive to the human T-cell lymphotropic virus type I transcriptional activator, Tax”. Mol. Cell. Biol. 14:456, 1994; Adya, N., L.-J. Zhao, W. Huang, I. Boros, and C.-Z. Giam: “Expansion of CREB's DNA recognition specificity by Tax results from interaction with Ala-Ala-Arg at positions 282-284 near the conserved DNA-binding domain of CREB”. Proc. Natl. Acad. Sci. USA 91:5642, 1994).